This invention relates to a rolling method and a rolling mill for an ingot originating from a continuous casting machine of the wheel-and-belt type.
The cross-sectional shape and geometry of an ingot leaving a continuous casting machine of the wheel-and-belt type are governed substantially by two factors. In the first place, the ingot has a necessarily flat upper face because of its formation in contact with the metal belt, which, in a casting machine of the said type, closes the mould in the casting wheel over a certain arc, and in the second place it comprises two lateral faces formed in contact with the lateral walls of the casting mould, which must necessarily lie at a certain draft angle to the normal to the upper flat face to enable the ingot to be separated from the casting wheel.
The draft angle, normally 8.degree. to 10.degree., cannot vary substantially, and thus the only freedom open to the designer in modifying the cross-section of the ingot to any degree in order to better adapt it to the requirements of the rolling to which the ingot will subsequently be subjected, is in the choice of the height/width ratio of the cross-section, and the extent of rounding of the side opposite the flat face, this side reproducing the shape of the base of the casting mould.
In the continuous rolling of these ingots, various methods have been proposed for the first rolling passes in consideration of the particular shape of their cross-section. The most known and widely used methods are those which use several successive rolling stands each with three rolls disposed at 120.degree. to each other to define substantially hexagonal/triangular or round/triangular alternate passes, or several rolling stands each of two rolls with substantially oval/round alternate passes.
In my U.S. Pat. No. 4,044,586 there is proposed a particularly advantageous method for rolling an ingot originating from a continuous casting machine by using three-roll rolling stands disposed directly downstream of the continuous casting machine.
With two-roll stands, and in particular in the case of an oval pass, because of the substantially trapezoidal cross-section of the ingot, there is the drawback of a particularly evident amount of edge outflow of the rolled material, deriving from the compressing of the flat major side of the ingot cross-section during the first rolling pass, with the formation of an excess of material at the edges adjacent to the major side of the cross-section, which project outwards to a marked degree.
This edge outflow is impossible to avoid or even control. Because of the fact that the outflown material again becomes rolled in the second rolling stand, the rolls of which are disposed at 90.degree. to the rolls of the first stand, defects arise known as over-rolling in the case of wire rod. In such a case the projecting edges become bent back on themselves and compressed during the second pass, but without becoming integrated with the underlying material, neither in that pass nor in the subsequent passes. This over-rolling defect leads to frequent wire breakage when the rod is drawn to a diameter of less than 0.5 mm.
As the amount of outflow at the upper edges of the ingot after the first rolling pass is proportional to the increase in the percentage reduction of the cross-section during the said rolling pass, it is not possible to utilize the first rolling pass to the maximum extent which could be supported by the type of metal being rolled.